As fillers to give conductivity to polymer materials, conductive carbon materials such as acetylene black and carbon nanofiber (hereinafter, referred to as CNF) and mixtures thereof are used. In particular, use or addition of CNF is characterized in that high electric conductivity is obtained with relatively low conductive filler content. Such characteristics raise expectations. The CNF here generally has a fibrous shape with an outer diameter from 5 to 100 nm and an aspect ratio, indicating a ratio of a fiber length to the outer diameter, of 10 or more.
Dispersibility of CNF is important to use the CNF as a filler to give conductivity to a matrix, such as a polymer. Fine carbon fiber such as conventional CNF unfortunately has poor dispersibility due to a secondary structure formed by the fiber complexly entangled with each other. Therefore, production of a highly conductive polymer material by sufficiently dispersing such CNF requires a step, for example, of composition with a polymer material using a solvent (PTL 1).
PTL 2 describes a conductive composite material that is capable of efficiently exhibiting conductivity even when a small amount of CNF is added. PTL 3 describes a conductive polymer material produced by melt kneading CNF excellent in dispersibility and a thermoplastic polymer material. PTL 4 describes improvement in dispersibility and kneadability of CNF by composition with a polymer or powder.